Heart rate detection from plantar bioimpedance measurements

In this paper, a novel technique for heart rate measurement on a standing subject is proposed that relies on electrical impedance variations detected by a plantar interface with booth feet, such as those in some bathroom weighting scales for body composition analysis. Heart-related impedance variations in the legs come from arterial blood circulation and are below 500 m Omega. To detect them, we have implemented a system with a gain in excess of 600, and whose fully differential AC input amplifier has a gain of 4.5 and a common-mode rejection ratio (CMRR) higher than 90 dB at 10 kHz. Differential coherent demodulation based on synchronous sampling yields a signal-to-noise ratio (SNR) of about 54 dB. The system sensitivity is 610 mV/Omega. The technique has been demonstrated on 18 volunteers, whose bioimpedance signal and ECG were simultaneously recorded. A Bland-Altman plot shows a mean bias of -0.2 ms between the RR time intervals obtained from these two signals, which is negligible. The technique is simple and user friendly and does not require any additional sensors or electrodes attached to the body, hence no conductive gel or skin preparation.